1. Field of the Invention
The present invention relates to a multistage air pump, especially to an air pump that can pressurize air inside the air pump in multiple stages within a single push by a piston.
2. Description of the Prior Art(s)
Air pumps are common tools used in daily life. The air pumps are used for inflating articles such as balls or tires, especially the tires of mountain bicycles.
With reference to FIG. 15, a first conventional air pump comprises a cylinder 12, a piston assembly 11, and an exhaust seat 13. The cylinder 12 is tubular and has a front cap 121 and a rear cap 122 respectively mounted on two opposite ends of the cylinder 12. An inlet one-way valve is mounted in the front cap 121 of the cylinder 12 and only allows air to flow into the cylinder 12. The rear cap 122 has a through hole 123. The piston assembly 11 has a piston rod 111, a handle 112, a piston 113, and a sealing ring 114. The piston rod 111 is mounted through the rear cap 122 and axially protrudes in the cylinder 12. The handle 112 is mounted on an outer end of the piston rod 111. The piston 113 is mounted on an inner end of the piston rod 111. The sealing ring 114 is mounted around the piston 113. The piston 113 and the sealing ring 114 are disposed inside the cylinder 12 and are driven to move forward and backward alternately by the piston rod 111. A front chamber is defined in the cylinder 12 and between the piston 113 and the front cap 121. A rear chamber is defined in the cylinder 12 and between the piston 113 and the rear cap 122. The exhaust seat 13 is mounted in the front cap 121 of the cylinder 12.
During operation, the exhaust seat 13 is connected to an inflatable article. A user holds the handle and alternately pushes and pulls the piston assembly 11 to drive the piston 113 and the sealing ring 114 to move forward and backward alternately. When the piston assembly 11 is pulled backwardly, air outside the cylinder 12 is drawn into the front chamber of the cylinder 12 via the inlet one-way valve, and air inside the rear chamber of the cylinder 12 is pushed to flow out of the rear chamber via the through hole 123 of the rear cap 122. When the piston assembly 11 is pushed forwardly, the air inside the front chamber of the cylinder 12 is pressurized and then flows out of the front chamber via the exhaust seat 13 to inflate the inflatable article. However, although the first conventional air pump has a simple structure and is easy for operation, the first conventional air pump can only provide low pressure air. Thus, the first conventional air pump can only be used for inflating articles that need low pressure air, such as balls. The first conventional air pump cannot be used for inflating articles that need high pressure air, such as tires of bicycles.
With further reference to FIG. 16, in order to inflate the tires of the bicycles, especially the tires of the mountain bicycles, a second conventional air pump is designed. The conventional air pump comprises a cylinder 22, a piston assembly 21, and an exhaust seat 23. The cylinder 22 has a front cap 221 and a rear cap 222 respectively mounted on two opposite ends of the cylinder 22. An inlet one-way valve is mounted in the front cap 221 of the cylinder 22. The rear cap 222 has a through hole. The piston assembly 21 has a piston rod 211, a handle 212, a piston 213, and a sealing ring 214. The piston rod 211 is mounted through the rear cap 222. The handle 212 is mounted on an outer end of the piston rod 211. The piston 213 is mounted on an inner end of the piston rod 211. The sealing ring 214 is mounted around the piston 213. The piston 213 and the sealing ring 214 are driven to move forward and backward alternately by the piston rod 211. The exhaust seat 23 is mounted in the front cap 221 of the cylinder 22.
Compared with the first conventional air pump, a volume of the cylinder 22 and a cross-sectional area of the piston 213 of the second air pump are reduced. Accordingly, an inner diameter of the cylinder 22 of the second conventional air pump is shorter than an inner diameter of the cylinder 12 of the first conventional air pump. The second conventional air pump has higher compression ratio than the first conventional air pump, and can provide high pressure air to inflate the tires of the bicycles. However, since the volume of the cylinder 22 of the second conventional air pump is reduced, each time when the user pushes the piston rod 211, only a small amount of the high pressure air is formed. Therefore, for inflating the same article, times of pushing and pulling the piston assembly 21 of the second conventional air pump are more than times of pushing and pulling the piston assembly 11 of the first conventional air pump. Inflating the article with the second conventional air pump takes extra labor work and increases operation time, which leads to low efficiency.
To overcome the shortcomings, the present invention provides a multistage air pump to mitigate or obviate the aforementioned problems.